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Monday, December 14, 2015

I receive the replacement CDSH270 Diodes for my Antenna Tuner today, with a little work and with a TUIT they were installed. The Tuner now works as before.

While during the surgery on the tuner, I installed some sharp pointed spark gaps at the SO-239 connectors, and between the Balanced Line Terminals. Maybe they will help save the Diodes the next time I forget to disconnect the Antenna during the an Electric Storm.

For the spark gaps, I soldered one end of a heavy copper wire directly to the SO-239 RF center connector, and then bolted the other end directly to ground (at the connector mount point). After the "Short" was installed, I cut it at the center with dull side-cutters, leaving sharp points on each side. The gap was adjusted to about 1/20 of a inch. The same was done between the Balanced Terminals.

With another TUIT, I installed a 6 AWG ground wire to the Tuner, but waiting for a large spade lug to make the final connection. My available lugs are not big enough for 6 AWG.

Today was nice and sunny, so I also raised the Long Wire Antenna to it normal height. Yesterday when I was repairing it, it was too cold and wet to futz with it to bring it to full height at 90 feet AGL.

Two years ago I tried to work on the antenna, but could not get the 3/32 support line to slide back down the tree. I suspected it had "grown" into a limb, or was pitched into a warren slot. My friend Jeff - KO7M and I was able to pull together hard enough to finally break it free. To avoid the same problem again, I slipped the line into a 20 foot piece of 1/4 inch polypropylene ice-make tube. which we pulled up into the tree where the support line goes over the branches. Now two years later the line was easy to pull down and then back up.

So far I have made a few SSB contacts with the current antenna configuration. Now I am anxious to work some WSPR - but another TUIT is in the way.

Sunday, December 13, 2015

I removed the 9:1 BalBal from my Off-Center-Fed Long Wire Antenna, as described below, it did not work as well as expected. More testing and measurement needed.

There has been a lot of chatter on the local Yahoo pQRP email list about building some 9:1 Ununs as a group project. I already have the Toroids and Wire and had planned to make several of my own - I was just waiting for a Round-TUIT (TUIT).

For the last several (6 or so) years, I have been using a 300 foot Off-Center-Fed (OCF) Long Wire antenna. It is strung up near the top of some nice tall trees with the feed point at about 90 feet AGL. It has worked very well on all bands for WSPR, QRSS, CW, and general Ham use. The antenna is fed with 450 ohm ladder line fed from a Tuner with an internal Balun. I rarely have to provide much L or C adjustment for reasonable performance.

With a crippled antenna (see below), last night I worked Sandy - KG7FFP as she is new General License Ham - Good going Sandy! We were on 80m, 3.885MHz.

There are several things that I have wanted to try, to see if I could improve performance.

The first, is a no-brainer; Ground the Tuner and the Rig, something that has always been waiting for another TUIT.

The second, may be more controversial, I have wanted to put a 9:1 BalBal at the feed point of the OCF Long Wire. With 450 ohms ladder line in, the output should approximate 4050 ohms, which I think would more closely match typical OCF Long Wire feed impedance. In my case the short end of the Long Wire is about 20 feet. The Feed Line drops straight into my Loft's window, across the celling to the Ham Rig, I think more impedance is desirable, in reality, I thing everything in close proximity becomes the antenna, that is; the long wire, feed line, metal roof, and the moss soaked wet-trees, and I don't think they are typically just 450 ohms :-)

Another "nit" that I wanted to improve (or remove) is the "egg" insulator at the feed point. I have never really liked this centre point set up, but it is what I had when it was first installed. The new set up is a now a 14 inch long, 3/4 inch PVC (conduit) with three holes in each end. The antenna wire goes through a "Z" bend at each end of the conduit, then "V's" down to my new 9:1 BalBal, which is about 18 inch lower. The antenna wire is standard, solid copper 12 AWG conduit wire.

My antenna tuner is still in my eShop waiting replacement diodes (CDSH270), they are on order and should arrive soon. The antenna and Tuner took a near hit during one of the last electric storms. I was near the radio and heard a snap or spark across the terminals or somewhere inside, the radio did not suffer any ill effects, the tuner TOOK it ALL.

So, now what?

My newly configured OCF Long Wire Antenna is now up and ready for use, but still waiting tuner repair.

I am expecting some VERY GOOD to EXCELLENT performance from this antenna configuration, because it was installed exactly as per the weather-man specification - that is, it was raining all the time I was outside working with; support lines, antenna wire, BalBal, and feed line. I got soaked ! Everyone knows, that the best antennas always go up during inclement weather.

But, time will tell, I will post my impressions, measurement, and results, when available.

Tuesday, December 8, 2015

If you have been following my recent blog post, I posted about using "curl" to send SMS messages to a Cell Phone Number, I have also created a software function to send a similar SMS messages from Esp8266, and/or using the same strategy this could be used on a Raspberry PI. The SMS messages are transferred to the "textbelt.com" Open SMS Gateway. Access is via a HTML URL post, or via a command line "curl" command which simulates a HTTP post. The textbelt.com server imposes some limits, that is; no more than 3 messages per 3 minutes, and no more than 75 messages per day from a single IPA.

The folks at Textbelt are gracious to allow us "small-time" users to access their gateway for FREE, - SPAMMERS are not welcome.

But, if you want your Raspberry PI or Esp8266 to send an actual "email", that is another problem. Accessing an "Open" Simple Mail Transport Protocol Gateways (an SMTP) is normally a problem for most users. The reason an "Open" SMTP is desirable for our Internet-of-Things (IoT's) is that authentication is NOT required on Open SMTP's, and therefore there is NO need for a "clear text" password saved in a file, nor is it transferred over the Internet.

There are a few Open SMTP available, where services are purchased by SPAMMERS to do their thing. A few of these SMTP gateways allow low-volume users access to their services for FREE, with some restrictions. I have tried several Open SMTP gateways, and found most have additional requirements that I did not agree, like; sign up with a credit card.

I am currently using "smtp2go.com" which has a FREE Option and does not require a credit card to sign up. For low volume users like myself, it has a limit of 25 emails per hour, and/or 1000 emails per month, - which is more than enough email for my IoT's to report status, and/or other reports to me via email.

I have implemented a Mail Transfer Agent (MTA) which "talks" to the Open SMTP to send my messages. Messages can be sent to my normal Gmail Account, or to my Cell Phone as an SMS text message, or to any other email address that I desire.

I have written complete MTA function for my use with my ERB-EspWebServer Farm, but provided just some excerpts here for discussion. At some point I will post the complete listing on my GitHub.

Sorry, I did not include the "set up", but I think the upper case "#define" are obvious.

As can be seen, the logic of the MTA is really very simple: send a message, and then wait for a response, repeat as necessary, stop on error or finished.

Note: For the Raspberry IP, this can be implemented as a Bash script, in fact the details for the above program were first worked out using a script on a Linux Workstations. I will post the script later.

What can make it difficult and secure is that VERY large numbers can be used, and therefore making it very difficult for an observer (villain) to use brute force to discover the Shared Secret Key.

If you read the available documentation about Diffie Hellman Key Exchange, you will no doubt learn all about; Alice, Bob (the good guys), and Eve (the villain). I suggest it for some bed time reading.

Note: the online documentation suggest that Alice and Bob must first each know some public numbers for "g" and "p".

But, there is nothing preventing a proper and secure Key Exchange when Alice initiates the process by sending Bob three numbers; "g", "p" and "g2B" (g to Bob) values.

Bob would need to use these three values to compute the "Shared Secret Key". Bob also returns to Alice a computed "g2A" (g to Alice), now Bob is finished, he now knows the Shared Secret Key. Alice will also know the Shared Secret Key when she completes her next calculation.

Alice uses the returned "g2A" value to compute the same "Shared Secret Key".

It looks like magic, but it is simple when you look at the maths with small numbers, and knowing:

You will also learn from online documents, that "p" should be a Large Prime Number, and "g"
should be greater than "0", but less than "p", and "k" is a Secret Key
that should NOT be shared.

Which is all good advice, if you are implementing a banking system, or saving the world via encrypted text messages. But the Diffie Hellman function works regardless of the numbers used, so in the examples below, some small and maybe inappropriate values are used.

What started my interested in Diffie Hellman Key exchange is the UNIX provided, and neat little and powerful, utility called "bc". The "bc" command can deal with VERY large numbers without difficulty.

An Example:

A Diffie Hallman Example, with small numbers.

Assume that Alice, Bob and Eve will know and/or use the following:

gP =16pP = 101

As stated above, these numbers could be suggested by Alice or Bob, or both. Or, the numbers could be hard coded into a application that they both use. If large enough, the knowledge of these numbers does NOT detract from security.

Note: For this example, I will use the upper case "P" to denote a number is "Public", "pP" is normally prime. I will use "kA" for Alice secret key, and "kB" for Bob's secret key. Also, "g2B" and "g2A" are the computed exchanged values. Then finally, "gS" becomes the value of the "Shared Secret Key".

Note: "g2B" and "g2A" are just like other "g" values that can be used in the same Diffie Hellman formula.

Eve can eavesdrop on all of the network data exchanges between Alice and Bob, but will NOT be able to compute the Shared Secret Key, because Eve is not privy to Alice's or Bob's Secret Keys.

-----------------------

Alice's Secret Key, known only by her, is: kA = 7

Alice wants to Share a Secret Key with Bob.

Alice computes a: g2B = (gP ^ kA) mod pP with her key.

Alice will do this on UNIX system with the values above using the following command:

$ g2B=`echo "(16^7)%101" | bc`

g2B becomes 80

Alice can now send "qP", "pP" and "g2B" to Bob as clear text within an email (or whatever). Note: Eve may or can eavesdrop on the Internet traffic.

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Bob's Secret Key, known only by him, is: kB = 11

Because Bob also uses a UNIX system, he computes from the values sent from Alice a value to be returned to Alice: g2A = (gP ^ kB) mod pP

$ g2A=`echo "(16^11)%101" | bc`

g2A becomes 71

Bob can now sends Alice "g2A", so she can complete her side of the key exchange.

With the value of "g2A" sent by Bob, Alice can complete her side of the exchange, by computing: gS = (g2A ^ kA) mod pP

$ gS=`echo "(71^7)%101" | bc`

gS becomes 54

Alice now finished, she knows the Shared Secret Key as "54".

-----------------------

Both Alice and Bob both know the same value of "gS", the Share Secret Key as "54". Alice or Bob, could now use the Shared Secret Key to encrypt messages that hopefully only the opposite could decrypt.

Eve is left clueless.

-----------------------

But there is a flaw !

With small numbers, as used in this example, Eve could Brute Force a decryption solution, by trying all numbers starting with "0". In this case it will only take 54 iterations to find the correct Shared Secret Key that would successfully decrypt Alice's message. Eve could easily know (via eavesdropping) the value of "pP" is small, and therefore, can assume the Secret Key is some number between 0 and 101, which implies a brute force attack is warranted.

But Normally

Normally, the values of "gP", "pP", and the resulting "g2B" and "g2A" are VERY much larger numbers, maybe hundreds or thousands of digits.

Imagine what Eve prospects of having enough CPU horse power to attempt a sequential attack if "pP" is extremely large number - prospects = none.

If "pP" is large enough, a brute force attack could take forever even with multiple very fast computers.

Some Experiments

Remember I said "bc" can deal with large numbers, well, to avoid "typing" a large number I will use exponentiation to generate some numbers for "qP" and "pP", for Alice try this: